Process for the production of alkali metal and alkaline earth metal borohydrides



United States Patent PROCESS FOR THE PRODUCTION OF ALKALI METAL AND ALKALINE EARTH METAL BORO- HYDRIDES Roland Kiister, Mulheim (Ruhr), Germany, assignor to Studiengesellschaft Kohle m.b.H., Mulheim (Ruhr), Germany No Drawing. Filed Apr. 28, 1958, Ser. No. 731,125 Claims priority, application Germany Apr. 30, 1957 7 Claims. (Cl. 23-14) This invention relates to a process for the production of alkali metal and alkaline earth metal borohydrides.

Belgian patent specification No. 559,063 discloses a process for the production of alkali metal and alkaline earth metal bor-ohydrides from borazanes having 3 hydrocarbon radicals, especially N-trialkyl borazanes, by reaction With an alkali metal or alkaline earth metal hydride, with a metal compound of the general formulae MX or MX in which M is an alkali metal or alkaline earth metal and X is a radical R or OR, in which R represents an aliphatic, aromatic or hydroaromatic hydrocarbon radical, or with a complex salt of one of these compounds with a boric acid ester. If metal alcoho-lates or metal hydrocarbons, especially metal alkyls, are used for the reaction, then boric acid esters or boron trihydrocarbons are obtained as secondary products.

X represents H, R or OR; R represents a hydrocarbon radical.

This process yields the metal borohydrides very smoothly and with a high degree of purity. The process has however certain disadvantages, since the startingrnaterials are almost always difficult to prepare. For example, the alkali metal and alkaline earth metal hydrides are advantageously obtained from the corresponding metals themselves by treatment with hydrogen, and therefore they require the use of the pure metals, such as sodium or calcium metal, so that the production of the sensitive hydrides is not in any case a reaction which is commercially very convenient. Although other processes have recently been proposed for the production of alkali metal and alkaline earth metal hydrides which do not start from the metals, these recently proposed processes operate at inconveniently high temperatures. The alkali metal and alkaline earth metal alcoholates are certainly more readily available than the hydrides, but, as already apparent from the equations set out above, it is not possible with the aid thereof to convert all the boron of the borazane into a complex metal borohydride. The same applies as regards the use of alkali metal and alkaline earth metal alkyls, and in addition these metal alkyls are almost always less readily available than the hydrides or alcoh-o lates.

However, this last premise must be qualified: a few organometallic compounds of the alkali metals and alkaline earth metals exist which are very readily available, namely the compounds of these metals with acetylene, especially the compound calcium carbide but also substances such as for example NaC H or Na c It therefore seems obvious to split up the N-trialkyl borazanes by reacting them with these acetylidcs. It has however been found that such a splitting reaction is not readily possible.

It has now been found that N-trialkyl borazanes can be split up by reacting them with alkali metal or alkaline earth metal acetylides in the presence of hydrogen under pressure, and accordingly the present invention provides a process for the production of alkali metal and alkaline "ice earth metal borohydrides, which comprises reacting an N-trialkyl borazane with an alkali metal acetylide or alkaline earth metal acetylide in the presence of hydrogen under pressure. For example, N-triethyl borazane can be split up at temperatures higher than C., preferably between 200 and 360 C. by calcium carbide, the acetylide fraction of the calcium carbide becoming free mostly in the form of ethane. The temperatures used are so low that the initial formation of calcium hydride from calcium carbide must be considered as not occurring in practice, since the temperatures at which calcium carbide reacts with hydrogen alone to form calcium hydride are substantially higher. Moreover, calcium carbide only reacts easily with hydrogen to form calcium hydride if certain catalysts are present, and in addition the calcium hydride formation requires a substantially higher temperature than is necessary in the process of the invention. The explanation of the surprising technical effect obtained by using the process of the invention must be sought in the fact that the metal acetylides, in conjunction with the borinc fraction of the borazanes, initially give complex compounds such as the following for the simplest case of the complex compound of monosodium acetylide and borinc:

N2l [H BCECH] This compound probably dissolves in the borazane, and the triple bond is then saturated with hydrogen, whereafter the saturated organic boron hydride which is formed is split up by reaction with hydrogen.

If the process of the invention is combined with other known reactions, it is for example possible with the sole use of boric acid anhydride, calcium carbide and hydrogen to produce at will either calcium borohydride or diborane, the borazane starting material used in the process of the invention merely representing one very convenient reaction promoter.

The process of the invention can also be carried out by hydrogenating a boron alkyl in the presence of calcium carbide and a tertiary amine. In this case, the N-trialkyl borazane is formed in situ, and this particular embodiment of the process of the invention leads to calcium hydride in one operation and in one and the same reaction vessel via the N-trialkyl borazane.

The following examples further illustrate the invention.

A finely ground calcium carbide suspension consisting of 12.8 g. (0.2 mol.) of calcium carbide in 50 cc. of perhydrocumene is introduced together with 30 cc. of tetrahydrofurane and 46 g. (0.4 mol.) of N-tricthyl bora- Zane into a 300 cc. autoclave. Hydrogen is introduced to give a pressure of 250 atm. and the autoclave is heated, while shaking, to 230240 C., the pressure falling over a period of 6-8 hours to a constant final value. After cooling, the gas (excess hydrogen and 6 g. of ethane) is vented, the contents are removed from the autoclave and the solution (light yellow in colour) is filtered oil and concentrated by evaporation. After distillation under reduced pressure and drying, a completely colourless calcium borohydride is obtained (yield 13.5 g.=96.5% of the theoretical).

Example 2 in amino From 64 g. (1 mol.) of calcium carbide in a mixture of 24-0 g. (2 mols.) of boron tri-n-propyl and 220 g. (2.18 mols.) of tricthylaniinc (previously finely ground in a vibrating ball mill) in a 1 litre autoclave, calcium borohydride is obtained in a substantially quantitative yield over a period of 5 hours by introducing hydrogen to give a pressure of 350 atm. and heating to a maximum temperature of 260 C. After the pressure has reached a constant final value, the autoclave is cooled and the gases vented. The calcium borohydride does not generally require purification, though recrystallisation from tetrahydrofurane yields a completely colourless product.

A suspension of 48 g. (1 mol.) of sodium acetylide in 115 g. of N-triethyl borazane and 100 cc. of perhydrocurnene is placed in a 500 cc. autoclave. Hydrogen is introduced to give a pressure of 200 atm. and the autoclave is heated to 200-230 C., the pressure falling to a constant value over a period of 81[) hours. The contents of the autoclave are cooled and the gases (excess hydrogen and ethane) vented, whereafter the suspension obtained is removed from the autoclave. After the liquid has been distilled off, sodium borohydride (38 g.) is obtained as residue; the yield is quantitative.

Example 4 32 g. (0.5 mol.) of finely ground potassium acetylide in 150 cc. of decahydronaphthalene are introduced together with 37 g. of N-trimethyl borazane into a 500 cc. autoclave. After hydrogen has been introduced to give a pressure of 150 atm., the autoclave is heated for hours at 170-200 C., the pressure then falling. On completion of the reaction, the excess hydrogen is vented together with the hydrocarbon gas. There remains a suspension of potassium borohydride in decahydronaphthalene and the amine which is formed. After the liquids have been distilled off, g. of potassium borohydride are left; this can be recrystallised from Water.

Example 5 What I claim is:

1. A process for the production of compounds selected from the group consisting of alkali metal and alkaline earth metal borohydrides, which comprises reacting an N- trialkyl borazane with a compound selected from the group consisting of alkali metal acetylides and alkaline earth metal acetylides in the presence of. hydrogen under pressure in the absence of a catalyst and at a temperature not exceeding 300 C.

2. A process according to claim 1, wherein calcium carbide is used as alkaline earth metal acetylide.

3. A process according to claim 1, wherein sodium acetylide NaHC is used as alkali metal acetylide.

4. A process according to claim 1, wherein temperatures between and 200 C. are used.

5. A process according to claim 1, wherein temperatures between 200 and 300 C. are used.

6. A process according to claim 1, wherein the N- trialkyl borazane is produced in situ from a boron alkyl and a tertiary amine.

7. A process according to claim 1, wherein said acetylide is a compound having the formula M H C wherein M represents a member selected from the group of alkali metals and alkaline earth metals, x is a whole number of from 1 to 2 (the valence of M) and y is a Whole number of from 0 to 1, wherein, when x:2, 31:0.

References Cited in the file of this patent UNITED STATES PATENTS (February 7, 1957).

Boron Compounds: More spotlighted Than Ever, Chemical Engineering, vol. 64, May 1957, page 166.

Thorpe et al.: Thorpes Dictionary of Applied Chemistry, 4th edition, 1937, vol. 1, page 73. 

1. A PROCESS FOR THE PRODUCTION OF COMPOUNDS SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL AND ALKALINE EARTH METAL BOROHYDRIDES, WHICH COMPRISES REACTING AN NTRIALKYL BORAZANE WITH A COMPOUND SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL ACETYLIDES AND ALKALINE EARTH METAL ACETYLIDES IN THE PRESENCE OF HYDROGEN UNDER PRESSURE IN THE ABSENCE OF A CATALYST AND AT A TEMPERATURE NOT EXCEEDING 300*C. 